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eintr: Truth & Falsehood

 
 3.9 Truth and Falsehood in Emacs Lisp
 =====================================
 
 There is an important aspect to the truth test in an ‘if’ expression.
 So far, we have spoken of “true” and “false” as values of predicates as
 if they were new kinds of Emacs Lisp objects.  In fact, “false” is just
 our old friend ‘nil’.  Anything else—anything at all—is “true”.
 
    The expression that tests for truth is interpreted as “true” if the
 result of evaluating it is a value that is not ‘nil’.  In other words,
 the result of the test is considered true if the value returned is a
 number such as 47, a string such as ‘"hello"’, or a symbol (other than
 ‘nil’) such as ‘flowers’, or a list (so long as it is not empty), or
 even a buffer!
 

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· nil explained               ‘nil’ has two meanings.
 

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